The technical field of the invention is that of the control of gearboxes for motor vehicles and more particularly that of the control of automatic gearboxes.
Controlling hybrid powertrains generally includes a function generating a drivetrain state target.
This function enables determination of a drivetrain state target optimizing the operating point of a hybrid powertrain.
It has to be remembered that a drivetrain state is defined by a combination of coupler state(s) and reducer state(s) specific to a given vehicle architecture.
For a gearbox of a vehicle with an internal combustion engine, one example of a drivetrain state is an engaged first reducer state and an engaged state of the clutch between the internal combustion engine and the gearbox. For a gearbox of a hybrid vehicle, one example of a drivetrain state is a disengaged state of the clutch between the internal combustion engine and the gearbox connected to the front wheels with electric motors propelling the vehicle via the rear wheels.
The inventors have based the design of the function generating a drivetrain state target on strategies for optimizing the operating point of a powertrain dedicated to vehicles with an internal combustion engine and enabling optimum management of the compromise between expected services such as acoustic factors, harshness, consumption and pollution reduction requirements.
The inventors went on to verify whether, if used in series, the current strategies could address the same requirement for hybrid powertrains. A preliminary study was therefore carried out to see if it was possible to make direct use of the strategies developed for powertrains dedicated to vehicles with an internal combustion engine to choose the most appropriate gearbox ratio or drivetrain state.
In the case of a hybrid powertrain, the significant differences as seen from the transmission side are as follows:                the internal combustion engine is no longer the only source of motive power,        for the same power requirement, there exist a multitude of possible combinations of the power delivered by the internal combustion engine and that delivered by the electric motor(s),        depending on the envisaged technical definition, the power of the electrical machine either passes through the transmission or does not,        the maximum and minimum static and dynamic limits of the hybrid powertrain can depend on the state of charge of the battery and can therefore vary as a function of time,        the electrical or ZEV (Zero Emission Vehicle) mode combines one or more specific possible drivetrain states by the same token as the discrete ratios.        
Thus analysis of the four services that these strategies have to provide in the case of a hybrid powertrain leads to the following conclusions:
The acoustic factors for the same operating point (speed, motive power) depend on the relative distribution of electrical power and thermal power. In other words, if only the electric motor is operating, the powertrain makes less noise than if the internal combustion engine and the electric motor are both operating. Compared to the electric motor, the internal combustion engine is the only source of noise.
Harshness, i.e. the performance of the powertrain, can depend on the state of charge of the battery.
Accordingly, when the battery is charged it is possible to use simultaneously the power delivered by the electric motor and by the internal combustion engine. On the other hand, if the battery is discharged the only available source of motive power is the internal combustion engine, which leads to a possible reduction of performance.
A new parameter has to be taken into account for the consumption and pollution reduction requirements. This refers to the energy management law the object of which is, in each of the possible powertrain states, to determine the relative distribution of the power delivered by the internal combustion engine and by the electric motor as a function of the state of charge of the battery.
The use of a hybrid powertrain therefore makes it necessary for the current strategies to evolve. The state of charge of the battery is an important new factor to be taken into account in generating the ratio target for a hybrid transmission.
Taking this factor into account makes it possible to optimize consumption and pollution reduction. In fact, the adoption of an electric powertrain was mainly motivated by reducing consumption, so taking same into account cannot be bypassed. It is therefore necessary for the strategies for generation of the drivetrain state target to interact with the energy management law.
Taking the battery charge state into account also makes it possible to optimize harshness. The variation of the harshness constraints as a function of the state of charge of the battery depends on the required performance of the powertrain. In fact, the maximum power of the powertrain depends on the power of the internal combustion engine or electric motor(s) present and available.
Taking the battery charge state into account finally makes it possible to optimize the acoustic factors. The impact on the acoustic factors of taking this state into account is less critical than the impact on consumption. By default, the engine speed targets could be calibrated for 100% internal combustion engine use. The resulting under-optimized zones would likely be minimal.
To summarize, the hybrid technology necessitates complete revision of the current strategies for controlling the optimization of the operating point of the powertrain (chosen gearbox ratio) that are generally oriented only toward application to the control of a vehicle with only an internal combustion engine. These strategies do not take into account the specific factors linked to the hybrid technology and notably the times and the conditions that govern changing from one drivetrain state to another.
There is therefore a requirement at this level.
The aim is for a motor vehicle equipped with an automatic gearbox to be in an optimum drivetrain state under all possible rolling conditions. Many constraints of NVH (Noise, Vibration, Harshness), reliability, mechanical ratio, brio (acceleration in reserve, driver demand, etc.) and other types make it possible to guarantee correct and appropriate behavior of the vehicle under the conditions applying when the driver seeks to maintain a stable speed or to accelerate.
When the driver's aim is to decelerate, the recommended drivetrain state is then a function of those same constraints, which in no way reflect the deceleration dynamic that the vehicle must exhibit. The drivetrain state will therefore potentially be irrelevant and could imply a “natural” level of deceleration (resisting force and engine braking) inappropriate to the road in use and its slope or the dynamic required by the driver via low or strong braking.
A concrete example of this problematic may be described with the aid of a vehicle with an internal combustion engine on a downgrade with a high coefficient. The vehicle and more particularly the strategies for choosing the optimum drivetrain state can tend toward the choice of a “long” ratio by complying with all NVH, brio (little acceleration in reserve), etc. constraints and being considered from the energy point of view as better than “short” drivetrain states. In this precise situation the vehicle can then become extremely skittish and may even accelerate, obliging the driver to brake strongly in order to decelerate or even to maintain the speed or even to downshift manually, obliging the driver to switch to manual transmission.
The technical problem to be solved is therefore as follows:
How to guarantee a certain level of deceleration of the vehicle by choosing the optimum drivetrain state?
The prior art includes the following documents.
FR2765652 describes applications to an automatic gearbox for non-hybrid vehicles in which it is possible to change only one ratio at a time with a braking assistance function conditional on depression of the brake pedal.
FR2875204 describes non-hybrid automatic gearbox applications with the braking assistance function conditional on depression of the brake pedal with control by increasing the static engine torque target.
FR2877416 describes non-hybrid automatic gearbox applications with the braking assistance function conditional on depression of the brake pedal with control by estimation of a primary rotation speed target.
US 20080046157 describes a strategy that functions only with determination of transmission ratios from speed thresholds that is valid only for vehicles where it is possible to change only one ratio at a time and that does not take into account differential forces that can arise from a headwind or a slope. The parameters making it possible to define a downshift are expressed as a speed offset relative to the existing lines and therefore with no concept of acceleration.
US 20140066251 describes a strategy that is valid only for vehicles where only one ratio can be changed at a time, which does not guarantee a given deceleration as it merely prohibits upshifts according to the given deceleration level of the vehicle (inhibition of all upshifts from a ratio N+2 or N+3) and does not take into account the differential forces that can arise from a headwind, a slope, etc.